Method of producing boron-doped monocrystalline silicon carbide

ABSTRACT

A CVD process or a sublimation process for doping an SiC monocrystal uses an organic boron compound whose molecules contain at least one boron atom chemically bonded to at least one carbon atom. Boron trialkyls are preferred organic boron compounds.

BACKGROUND OF THE INVENTION

The invention concerns a method of producing boron (B)-dopedmonocrystalline silicon carbide.

In addition to aluminum, boron is the most important dopant for p-typedoping of monocrystalline SiC semiconductor material.

In Applied Physics Letters, vol. 42, no. 5, Mar. 1, 1983, pages 460-462,a process is disclosed for producing a boron-doped layer ofmonocrystalline silicon carbide (SiC) of the 3C polytype (β-SiC) bychemical vapor deposition (CVD) on a monocrystalline silicon substrateat a temperature of 1400° C. In this known process, hydrogen (H₂) isused as the carrier gas, silane (SiH₄) is used as the precursor forsupplying silicon (Si) with a molar amount of 0.04% in the H₂ carriergas, and propane (C₃ H₈) with a molar amount of 0.02% in H₂ is used asthe precursor to supply carbon (C). The doping gas diborane (B₂ H₆) isadded to the gas mixture of the carrier gas and the two precursors inthe amount of 100 ppm in H₂ for doping. The deposited β-SiC layer has acharge carrier concentration of holes (p-type conduction) of 5.6·10¹⁴ to1.6·10¹⁵ cm⁻³.

In another process disclosed in Journal of the Electrochemical Society,vol. 133, no. 11, November 1986, pages 2350-2357, a boron-doped β-SiClayer is produced by CVD epitaxy at a temperature of 1360° C. withsilane (SiH₄) and ethene (C₂ H₄) as precursors, hydrogen (H₂) as thecarrier gas and diborane (B₂ H₆) as the doping gas. With an SiC layerproduced by this known method, only a small portion (0.2%) of the boronatoms introduced into the SiC are electrically active. To achieve a highcharge carrier concentration of the p-type conduction, the atomicconcentration of boron in SiC must therefore be so high that the surfacequality of the growing SiC layer is greatly impaired.

U.S. Pat. No. 4,923,716 discloses another CVD process for producingboron-doped β-SiC with diborane as the doping gas.

Furthermore, sublimation processes are also known for producingmonocrystalline SiC where an SiC bulk crystal of sublimed SiC is grownin the vapor phase (mainly Si, Si₂ C, SiC₂) on the wall of a vessel(Lely process) or on a seed crystal (modified Lely process).

Furthermore, plasma-assisted CVD process are also known for depositionof a boron-doped amorphous compound of silicon and carbon, a-Si_(1-x)C_(x) :H, with hydrogen inclusions on a substrate. For doping thea-Si_(1-x) C_(x) :H layer with boron, an organic boron compound with anunsaturated hydrocarbon residue or boron trimethyl or boron triethyl isadded to a precursor containing hydrogen gas and silane. The depositiontemperatures on the substrate are between 150° C. and a maximum of 300°C. The optical energy gap of the a-Si_(1-x) C_(x) :H layer is adjustedthrough the boron doping. Such a-Si_(1-x) C_(x) :H layers are used asp-type layers in a p-i-n solar cell based on amorphous silicon. Theamount of silicon in these known a-Si_(1-x) C_(x) :H layers is muchgreater than the amount of carbon (European Patent A 573,033 or PatentAbstracts of Japan, C-711, Apr. 19, 1990, vol. 14, no. 192 or Physica B,no. 170 (1991) pp. 574-576 or Journal of Non-Crystalline Solids, no.137+138 (1991) pp. 701-704 or Materials Research Society SymposiumProceedinqs, vol. 118 (1988) pp. 557-559 or Philosophical Magazine B,vol. 64, no. 1 (1991) pp. 101-111). Monocrystalline SiC layers cannot beproduced by these known methods.

SUMMARY OF THE INVENTION

The object of this invention is to provide a process for producingboron-doped monocrystalline silicon carbide, where the boron atomsincorporated into the silicon carbide crystal lattice have a high degreeof activation.

DETAILED DESCRIPTION OF THE INVENTION

This object is achieved according to this invention with the features ofClaim 1 or with the features of Claim 5.

In a CVD process for producing p-type boron-doped monocrystalline SiC,at least one precursor is used to supply silicon (Si) and carbon (C) andat least one organic boron compound is used for doping according toClaim 1. Each molecule of the at least one organic boron compound has atleast one boron atom that is chemically bonded to at least one carbonatom. In the monocrystalline SiC layer grown on a substrate by this CVDmethod, definitely more boron atoms are electrically activated than withthe boron-doped SiC layers obtained by the known methods. The ratio ofcharge carrier concentration of the holes (p-type conduction) to thechemical atomic concentration of boron atoms in the crystal lattice ofthe SiC layer is thus higher and may even be almost 1.

In a sublimation process according to Claim 5, solid silicon carbide(SiC) is at least partially sublimed, and monocrystalline SiC is formedfrom the sublimed silicon carbide in the vapor phase. At least oneorganic boron compound is used for doping the crystallizing SiCmonocrystals. Each molecule of the at least one organic boron compoundcontains at least one boron atom chemically bonded to at least onecarbon atom.

Boron atoms (B atoms) may be incorporated into the SiC crystal latticeas imperfections at silicon (Si) lattice sites, carbon (C) lattice sitesor interlattice sites. One boron atom at an Si lattice site iselectrically active and forms an acceptor energy level between about 250meV and 350 meV in the 4H polytype of SiC and between about 300 meV and350 meV in the 6H polytype of SiC. However, this acceptor level iscomparatively low, so a maximum of only about 1% of the boron atoms inthe SiC crystal are thermally ionized at room temperature. The electricactivity of boron atoms at C lattice sites has not yet been completelyelucidated. Perhaps the boron at these C lattice sites forms a lowenergy level whose contribution to the conductivity (donor or acceptor)has not yet been elucidated. However, it may be assumed that boron atomsat interlattice sites in the SiC crystal are essentially notelectrically active.

This invention is based on the finding that when using organic boroncompounds as dopants, the boron in the vapor phase in the molecules ofthe boron compound is chemically bonded to at least one vicinal carbonatom. Thus the incorporation of boron atoms at Si lattice sites in theSiC crystal lattice is greatly facilitated. However, boron atomsincorporated at Si lattice sites have the above-mentioned imperfectionenergy levels which contribute to the p-type conduction as acceptorlevels. This explains the high degree of activation of the boronimperfections incorporated into the SiC crystal layer by the processaccording to this invention.

The process according to this invention can be used to produce inparticular α-SiC of the 4H or 6H polytype, for example, with a p-typecharge carrier concentration of preferably about 10¹⁵ cm⁻³ to about5·10¹⁹ cm⁻³.

Advantageous embodiments and refinements of this process are derivedfrom the claims dependent upon Claim 1 and Claim 5.

One embodiment uses an organic boron compound whose molecules have atleast one branched or unbranched chain (aliphatic) hydrocarbon residue,such as an alkyl group or an unsaturated hydrocarbon group with doubleand/or triple bonds between at least some of its carbon atoms.

In another embodiment, the organic boron compound has at least one ringhydrocarbon residue, e.g., a cycloalkyl group or an aromatic hydrocarbongroup.

In an especially advantageous embodiment, at least one boron atom issurrounded by three carbon atoms in a generally tetrahedral bond in themolecules of the organic boron compound. Examples of such dopantsubstances include boron trialkyls such as boron trimethyl, borontriethyl, boron tripropyl, boron tributyl or boron tripentyl in allisomeric forms or aromatic compounds such as boron triphenyl. Thebonding of the boron atom in this embodiment in the vapor phasecorresponds to the subsequent bonding at an Si lattice site in the SiCcrystal. This facilitates the incorporation of boron atoms at Si latticesites in particular.

The hydrogen atoms in the hydrocarbon residues of the organic boroncompound may also be at least partially substituted, in particular byhalogen atoms.

Finally, organic boron compounds with several boron atoms in theirmolecules may also be used, in particular dimeric or trimeric boroncompounds where two or three boron atoms are chemically bonded to eachother. Then each boron atom is preferably bonded to at least one vicinalcarbon atom.

The organic boron compound is directed toward the developingmonocrystalline SiC, preferably with a carrier gas. A bubbling processmay be used with liquid boron compounds, such as the above-mentionedboron triethyl, boron tripropyl and boron tributyl. In the bubblingprocess, the liquid boron compound is poured into a container (cryostat)and the carrier gas is passed through the liquid and thereby enrichedwith boron. The boron-enriched carrier gas is used for supplying boronto the growing SiC monocrystal. The boron concentration in this carriergas enriched with the organic boron compound may optionally be furtherreduced by diluting with additional carrier gas. The preferred carriergas is hydrogen (H₂). However, a noble gas such as argon (Ar) or amixture of a noble gas and hydrogen may also be used as the carrier gas.

The vapor pressure of the liquid boron compound is preferably adjustedto a predetermined level by adjusting the temperature. Typically thetemperature in the liquid boron compound is between about -50° C. andabout +30° C. The boron dopant concentration in the growing SiCmonocrystal can be adjusted through the vapor pressure of the liquidorganic boron compound.

The boron dopant concentration in the SiC monocrystal can also beadjusted via the flow rate of the carrier gas in bubbling through theliquid organic boron compound. Typical flow rates for the bubblingcarrier gas are between about 1 and 100 ml/min. The flow rate of thecarrier gas can be adjusted with great accuracy with the help of a massflow controller, for example.

In the case of gaseous organic boron compounds, the boron compound issupplied to the growing SiC monocrystal either directly or diluted witha carrier gas.

In principle, any gas suitable for supplying Si and C in the vapor phasemay be used as the precursor (process gas) for the CVD process. In anadvantageous embodiment, a precursor mixture of at least one precursorcontaining silicon but no carbon, e.g., silane or disilane, and at leastone precursor containing carbon but no silicon, e.g., a hydrocarbon gassuch as methane, propane or acetylene (ethyne) is used. The C/Si ratioof carbon to silicon in the gas phase can be adjusted in this way.Preferred precursors in this embodiment include silane (SiH₄) forsupplying silicon (Si) and propane (C₃ H₈) for supplying carbon in thevapor phase. However, a precursor containing both silicon and carbon mayalso be used, such as the substance1,3-dimethyl-3-methylsilamethylenedisilacyclobutane or methylsilane.

In an especially advantageous embodiment, the precursors contain anexcess of carbon. Then the C/Si ratio is more than 1 in particular andless than approx. 10 in particular, preferably approx. 4. Due to theexcess carbon in the precursor mixture, the void concentration of Silattice sites is increased. This greatly facilitates the incorporationof boron atoms at Si lattice sites.

In the CVD process in general, the precursor or precursor mixture isdiluted with a carrier gas, typically in a ratio between approx. 1:35and approx. 1:100. Hydrogen (H₂) is the preferred carrier gas for the atleast one precursor. The ratio of silane to hydrogen can be adjustedbetween approx. 2·10⁻⁵ and 5·10⁻⁵. However, a noble gas such as argon ora mixture of hydrogen and a noble gas may also be used as the carriergas.

The mixture of precursors, organic boron compound and carrier gases isthen preferably conveyed into a recipient or combined there in the CVDprocess. At least one substrate is provided in the recipient. Theboron-doped SiC layer is grown on a crystallization surface of thesubstrate by chemical vapor deposition from the gas mixture. Allmaterials suitable for chemical vapor deposition of SiC may be used forthe substrates, including silicon and sapphire, but preferably SiC of acertain polytype such as 4H or 6H.

The growth temperatures at the crystallization surface of the substratefor the CVD process are generally set between 1000° C. and 2200° C., andfor producing α-SiC, specifically of the 4H or 6H polytype, thetemperature is generally set between 1500° C. and 2200° C., preferablybetween 1500° C. and 1700° C.

The amount of organic boron compound in the gas is preferably between10⁻⁸ and 10⁻⁶. The total gas pressure in the recipient is generally setbetween approx. 100 Pa and 2·10⁴ Pa, preferably between and approx.2·10⁴ Pa and 10⁵ Pa. When silane is the precursor, the ratio of organicboron compound to silane is preferably between approx. 10⁻⁶ and 10⁻³.

With the CVD process according to this invention, boron-dopedmonocrystalline SiC epitaxial layers can be produced, where the boronatom concentration increases essentially in proportion (linearly) to thegas partial pressure of the organic boron compound on the substrate inthe recipient, from approx. 10¹⁶ cm⁻³ at an organic boron compoundpartial pressure of 10⁻³ Pa (10⁻⁸ bar) up to approx. 10²⁰ cm⁻³ at anorganic boron compound partial pressure of approx. 1 Pa (10-5 bar).Thus, the boron atom concentration can be adjusted accurately over awide range.

In the sublimation process, the mixture of sublimed SiC in the vaporphase, organic boron compound and optional carrier gases is preferablyintroduced into a susceptor made of a heat-resistant material or theyare combined there. Crystallization of the SiC monocrystal with borondoping takes place in the susceptor. In an advantageous embodiment ofthe sublimation process, the boron-doped SiC monocrystal is grown on acrystallization surface of a seed crystal arranged in the susceptor. Allmaterials suitable for vapor-phase deposition of SiC can be used as theseed crystal, preferably SiC of a specific polytype such as 4H or 6H or3C. The seed crystal is preferably arranged in the susceptor. The growthtemperatures at the crystallization surface of the seed crystal in thesublimation process are generally between 1800° C. and 2500° C.,preferably between 2100° C. and 2300° C. In principle, all polytypes ofSiC can be produced by this sublimation process.

What is claimed is:
 1. A process for producing monocrystalline siliconcarbide doped with boron by chemical vapor deposition on a substrate,comprising the steps of:a) supplying silicon and carbon by conveying atleast one precursor into a recipient, b) supplying boron by conveying atleast one organic compound whose molecules contain at least one carbonatom chemically bonded to at least one boron atom into a recipient, c)setting a crystallization surface of the substrate to at least 1000° C.,d) forming a gas phase comprising the supplied silicon and carbon, e)establishing an excess of carbon compared to silicon in the gas phase,and f) growing monocrystalline silicon carbide on the crystallizationsurface from the at least one precursor and the at least one organiccompound.
 2. The process according to claim 11, wherein at least onesilicon compound that does not contain carbon is used as the precursorto supply silicon and at least one carbon compound that does not containsilicon is used as the precursor to supply carbon.
 3. The processaccording to claim 11, wherein the organic boron compound is related toa predetermined partial pressure, said partial pressure being set on thesubstrate between approximately 10⁻³ Pa and 1 Pa.
 4. A process forproducing monocrystalline silicon carbide doped with boron, comprisingthe steps of:a) at least partially subliming silicon carbide in itssolid form, b) forming monocrystalline silicon carbide from the sublimedsilicon carbide being in the vapor phase, and c) supplying boron usingat least one organic boron compound whose molecules contain at least onecarbon atom chemically bonded to at least one boron atom.
 5. The processaccording to claim 4, wherein monocrystalline silicon carbide is grownon a seed crystal.
 6. The process according to claim 1, wherein theorganic boron compound comprises a boron trialkyl or an aromaticcompound.
 7. The process according to claim 1, wherein the molecules ofthe organic boron compound contain at least one halogen atom.
 8. Theprocess according to claim 1, wherein the molecules of the organic boroncompound contain at least two boron atoms, each of which is chemicallybonded to at least one carbon atom.
 9. The process according to claim 1,wherein at least one carrier gas is passed through the organic boroncompound and the carrier gas enriched with the organic boron compound issupplied to a developing monocrystalline silicon carbide.
 10. Theprocess according to claim 9, wherein hydrogen is used as the carriergas.
 11. The process according to claim 9, wherein at least one noblegas is used as the carrier gas.
 12. The process according to claim 9,wherein a gas mixture of hydrogen and at least one noble gas is used asthe carrier gas.
 13. The process according to claim 9, wherein theconcentration of dopant for boron in the monocrystalline silicon carbideis adjusted by adjusting the flow rate of carrier gas through theorganic boron compound.
 14. The process according to claim 1, whereinthe concentration of dopant for boron in the monocrystalline siliconcarbide is controlled by adjusting the temperature of the organic boroncompound.
 15. The process according to claim 6 wherein the borontrialkyl is selected from the group consisting of boron trimethyl, borontriethyl, boron tripropyl, boron tributyl and boron tripentyl.
 16. Theprocess according to claim 6 wherein the aromatic compound is borontriphenyl.
 17. The process according to claim 15 wherein the aromaticcompound is boron triphenyl.